1. Field of the Invention
The present invention generally relates to a gain adjusting method and a gain adjusting device, and especially relates to a gain adjusting method and a gain adjusting device for adjusting gain of a voice signal to which signal processing is applied, and for outputting the gain-adjusted signal.
2. Description of the Related Art
The voice signal that includes speech and audio contents is often compression-coded to reduce information volume so that a smaller storage capacity when storing the contents in recording media and a lower transmission speed when transmitting the signal through a network such as the Internet and cellular phone systems may be utilized.
Further, various signal processing methods, such as a voice emphasis method, a noise canceller method, and an echo canceller method are typically used by IP telephone systems and cellular phone systems such that clarity of voice is enhanced. The signal processing methods tend to output a greater amount of power than the power of an input voice signal. For this reason, a gain adjustment process is provided so that the gain (level) of the output voice signal is properly controlled.
FIG. 1 is a block diagram of an example of a conventional gain adjusting device that includes a signal processing unit 1 for carrying out signal processing on an input voice signal, the signal processing including a voice emphasis method, a noise canceller method, and an echo canceller method; and a gain control unit 2 for controlling the gain (level) of the signal-processed voice signal so that the output level becomes equal to the input voice signal level.
In addition, Japanese Laid-Open Patent Publication No. 8-70228 discloses compensating for frequency characteristics of an audio signal, wherein the audio signal (an electric signal) is converted to voice and made audible, noise is extracted from the audio signal and a monitor signal of the voice, and the gain of the audio signal is determined in consideration of a masking effect based on the frequency spectrum of the noise.
An example is considered, wherein the voice emphasis processing is used in order to increase power of a formant, which is a peak of the frequency spectrum of the voice signal. That is, the power of the formants is increased as shown in FIG. 2A by the signal processing unit 1 of FIG. 1. When the power-increased spectrum is transformed into the time domain, the amplitude may exceed a limit, causing clipping to occur as shown in FIG. 2B, the clipping producing audible noise.
In order to prevent clipping, gain adjustment is performed by the gain control unit 2 of FIG. 1. Although generating of the noise by the excessive amplitude can be suppressed as shown in FIG. 3B, since the gain adjustment is conventionally carried out on the power of the whole spectrum, a problem is that the effect of the voice emphasis is reduced as shown in FIG. 3B.